EHSVs achieve good performance in steady state situations. However, problems have been identified in start-up transients during which spool valves have been slow to take up their commanded position. In some case of primary servos for the main and tail rotors of helicopters, this slow response can cause the Flight Control Computer (FCC) to shut down a hydraulic system.
Two conventional EHSVs are shown in FIGS. 1a and 1b respectively. FIG. 1a shows an EHSV with no electrical feedback while FIG. 1b shows an EHSV with electrical feedback. More specifically FIGS. 1a and 1b show a two stage ElectroHydraulic Servo Valve (EHSV). The EHSV 1 converts small electrical signals into proportional hydraulic flow. The first stage 2 consists of a torque motor 3, jet pipe 4 and receivers 5. The second stage 6 consists of a manifold 7 that houses the flow control valve 8 (i.e. the spool). Small electrical currents applied to the torque motor 3 cause the torque motor 3 to rotate the jet pipe 4 either to the left or to the right (as shown in the figure). These angular movements of the jet pipe 4 direct fluid from the high pressure supply PS (provided through pipe 13) and through either a first conduit 9 leading to a first end face 10 of the spool 8 or through a second conduit 11 leading to a second end face 12 of the spool 8. Any leakage at the jet pipe 4 passes through return line PR.
As the pressure on one end face (10 or 12) of the spool 8 is increased, the spool 8 is caused to move within the manifold 7. For example, if the jet pipe 4 is rotated so that it is directed to the left in FIG. 1 a, high pressure fluid is fed to first end face 10 of spool 8 which causes the spool 8 to move to the right. This movement of spool 8 permits high pressure fluid from supply PS to flow through first chamber outlet C1. On the other hand, if the jet pipe 4 is rotated so that it is directed to the right in FIG. 1a, high pressure fluid is fed to the second end face 12 of spool 8 which cause the spool 8 to move to the left. This movement of spool 8 permits high pressure fluid from supply PS to flow through second chamber outlet C2. Feedback wire 14 is a spring connected between the jet pipe 4 and to the centre of spool 8. As the spool 8 is driven towards its commanded position, it pulls the feedback wire 14 which in turn pulls the jet pipe 4 back towards the centre, balanced position once the desired position has been attained. All high pressure fluid that does not pass to the first chamber outlet C1 or the second chamber outlet C2 is drained through return circuit PR.
As illustrated in FIG. 1 a, the volume formed from first conduit 9 and first end face 10 is similar to the volume formed from second conduit 11 and second end face 12. Thus upon start up, the two opposed volumes fill/pressurize equally and the valve achieves a steady state quickly.
FIG. 1b shows an EHSV 1 with an electrical position feedback system 15. This system 15 supplements the closed loop feedback provided by feedback wire 14 and is also used for monitoring the current position of the spool 8 and for failure detection (e.g. by determining that spool position is not responsive to commands). However, as shown in FIG. 1b, the electrical position feedback system 15 is formed from a first sensor part 16 attached to the end face 10 of spool 8 and extending into second sensor part 17 which senses the axial movement of first sensor part 16 relative to second sensor part 17. The sensor parts 16, 17 may be of any form for detecting position, e.g. magnetic or capacitive. As can be seen in FIG. 1b, the volume of the fluid reservoir at end face 10 is significantly increased by the presence of the electrical position feedback system 15 compared with the volume of the fluid reservoir at the opposite end face 12. Moreover, this system 15 gives the reservoir a complex shape with a tortuous flow path for the fluid (e.g. oil) to flow around and through. Additionally, these areas can also be prone to air entrapment due to their blind nature. The effects of these features can result in a different time constant for the pressure at each end to stabilize upon rapid hydraulic start-up.